In studies of vision and audition, stimuli can be chosen to span the visible or audible spectrum; in olfaction, the axes and boundaries defining the analogous odorous space are unknown. As a result, the population of olfactory space is likewise unknown, and anecdotal estimates of 10,000 odorants have endured."

Source : PNAS, April 4, 2022

Put simply, there are currently no reliable methods to predict whether a newly made (or discovered) chemical compound will have a detectable odour or not. In addition, it's also difficult, often impossible, to predict what kind of smell a compound will have, even when very closely related to previously known and tested compounds.

This is primarily due to the lack of understanding about how perception of odours actually works (olfaction).

What makes a molecule have a smell? This simple question represents a significant gap in our understanding of olfaction."

[link as above]

Also see : Sense of smell - Olfactionplugin-autotooltip__plain plugin-autotooltip_bigSense of smell - Olfaction

It's known that humans have about 10 cm2 of olfactory epithelium, with roughly 5 million sensory neurons.

Smells are detected due to the activation of neurons by airborne volatile chemicals and particles. Different chemicals affect different groups of neurons, which, after cerebral processing, lead to the impression of 'smell'.